In radio communication systems, a mixer is used to up-convert a baseband (BB) signal or an intermediate frequency (IF) signal to a higher frequency, e.g. a radio frequency (RF) signal, for ease of transmission (when the mixer is used in a transmitter) or to down-convert a high frequency signal, e.g. an RF signal, to a lower frequency signal, e.g. a BB signal or an IF signal, for ease of signal processing (when the mixer is used in a receiver). The up-conversion or down-conversion is respectively performed by mixing the input signal of the mixer with a local oscillator (LO) signal generated by a local oscillator. In the receiver case, the RF signal is mixed with the LO signal in order to generate an IF signal or a BB signal.
In some mobile radio communication devices, the transmitter and receiver architectures are separated, i.e. separate circuitries are used for the receiver and the transmitter. However, in other mobile communication devices, a transceiver is used, which is a device that has both a transmitter and a receiver which are combined and share common circuitry. Transceivers normally include a duplexer which is a device that allows simultaneous bi-directional (full duplex) communication over a single channel. In radio communication systems, the duplexer isolates the receiver from the transmitter while permitting them to share a common antenna.
A challenge in modern radio communication systems has been, and continues to be, to design receivers (and transmitters) that can meet increasingly strict performance standards while fitting into ever shrinking packages. To this end, many modern radio receivers (and transmitters) are implemented on a single application specific integrated circuit (ASIC). One of these strict performance standards is the intermodulation requirement, in particular the so called second order intermodulation (IM2) requirement. Intermodulation can only occur in nonlinear systems. Nonlinear systems are generally composed of active components, meaning that the components must be biased with an external power source which is not the input signal (i.e. the active components must be “turned on”). However, even passive components can perform in a non-linear manner and cause intermodulation. Diodes or transistors are widely known for their passive nonlinear effects, but parasitic nonlinearity can arise in other components as well. For example, audio transformers exhibit non-linear behavior near their saturation point, electrolytic capacitors can start to behave as rectifiers under large-signal conditions, and RF connectors and antennas can exhibit nonlinear characteristics.
In the receiver case, a passive mixer generates IF or BB signals that result from the sum and difference of the LO and RF signals combined in the mixer. These sum and difference signals at the IF port are of equal amplitude, but generally only the difference signal is desired for processing and demodulation so the sum frequency (also known as the image signal) must be removed, typically by means of IF bandpass or BB lowpass filtering.
In the nonlinear case, further higher order components (caused by harmonics), like IM2 components, typically occur at the mixer output. The Second Order Intercept Point (IP2) is a measure of linearity that quantifies the second-order distortion generated by nonlinear systems and devices. At low power levels, the fundamental output power rises in a one-to-one ratio (in terms of dB) of the input power, while the second-order output power rises in a two-to-one ratio. When the input power is high enough and the device reaches saturation, the output power flattens out in both the first- and second-order cases. The second order intercept point is the point at which the first- and second-order lines intersect, assuming that the power levels do not flatten off due to saturation. In other words, the IP2 is the theoretical point on the RF input vs. IF output curve where the desired input signal and second order products become equal in amplitude as the RF input is raised.
Zero- and low-IF receiver architectures dominate today's low-cost wireless receiver market for Time Division Multiple Access (TDMA) and Time Division Duplex (TDD) systems. For Frequency Division Multiple Access (FDMA) systems and Code Division Multiple Access (CDMA) systems, like Wideband Code Division Multiple Access (WCDMA) systems, the strict IM2 and IP2 requirements typically necessitate more complex receiver solutions.
In a TDMA or TDD system, the wireless transmitter and receiver are not on at the same time but only in different, non-overlapping, time slots. Thus, for these systems the strongest receiver (Rx) interference is due to an external transmitter, picked up via the antenna of the TDMA or TDD system. In FDMA or in CDMA systems, like in a WCDMA system, the strongest Rx interferer is typically the wireless transmitter (Tx) itself, via leakage through the duplex filter of the system. Since the transmitter leakage at full power typically is >10 dB stronger than any external interferer, this will mainly set the IM2 and IP2 requirements.
For example, a WCDMA transmitter at +25 dBm power will result in a −25 dBm Rx signal when the dupJexer attenuation is 50 dB. If only −108 dBm static Rx interference (an interference that is present all the time) is acceptable, the receiver IP2 has to be >+44 dBm for the rectified Tx spectrum to be below the −108 dBm limit. For, e.g., GSM (Global System for Mobile Communications), the strongest interferer is 5 dB lower or −30 dBm, resulting in a 10 dB lower IP2 requirement for the same distortion levels.
Up till now, the common remedy for the high transmitter leakage levels has been to introduce a filter between the low noise amplifier (LNA) and the mixer of the receiver, typically an active mixer for noise reasons. Because of the small relative frequency separation between the closest Tx and Rx band edges, i.e. the duplex gap, this filter typically is a Surface Acoustic Wave (SAW) filter which can not be integrated into the transceiver ASIC, but has to be located on the printed circuit board (PCB) or module substrate, adding to the cost and complexity of the receiver structure.
Recently, alternating current (AC) coupling between the LNA and the mixer core has been employed as a means to enhance IP2 by blocking low-frequency IM2 noise to enter the mixer core, thereby preventing any leakage due to mixer imbalances.
A passive metal oxide semiconductor (MOS) mixer offers good performance in terms of noise and linearity, especially when its BB or IF port is at a virtual ground. The virtual ground eliminates the modulation of the mixer switches by the BB or IF signal which improves IP2. Due to the inherent nature of the MOS device, its switching threshold and channel conductance depends on the LO, RF and IF signals. These interdependences will generate cross products of these signals, including ones that cause IM2. By grounding the IF port, e.g. via a virtual ground, some of these cross products can be reduced resulting in less IM2 and consequently a higher IP2. Still the switching threshold will be modulated by the RF signal, resulting in an IM2 contribution in addition to that of the nonlinear channel conductance.
Today's AC-coupled mixer solutions provide enough performance when the duplexer isolation is 50 dB or better. For newer band configurations with smaller duplex gaps and thus less duplexer isolation this may not be possible. Also for cost reasons it may be advantageous to relax these duplexer requirements, e.g. by allowing duplexers with less duplexer isolation, by improving the mixer IP2.